Mechanical properties of large-diameter semi-grouted sleeve connections for railway platform canopies

To study the mechanical properties of a large-diameter semi-grouted sleeve of a railway platform canopy under a construction environment in winter, this study designed five groups of specimens for unidirectional tensile loading tests based on different curing conditions and freeze-thaw deterioration. The ultimate bearing capacity, strain distribution, failure mode, and grouting material damage were compared and analyzed under different working conditions. Based on the theory of thick-walled cylinders and the constraint analysis model, formulas for calculating the bond strength of grouted sleeve joints were developed to predict the bond strength of joints. Finally, a finite element model was established to study the effects of the steel bar embedded length, grouting material saturation, sleeve type, and steel bar diameter on the mechanical properties and damage of the spec-imens. The results indicate that the failure modes of GSCs include steel bar fracturing, steel bar slippage, sleeve fracturing, and screw thread failure. The ultimate bearing capacity of the GSC increased by 1.1% when the curing temperature increased from -5 to + 5 degrees C in the freeze-thaw group. Compared with the condition without freeze-thaw cycles, the short-term low-temperature curing freeze-thaw cycles increased the bond-strength of the grouted sleeve by 1.3%, whereas the high-temperature curing freeze-thaw cycles reduced the bond-strength by 3.5%. The maximum error between the proposed formula and the test result was less than 2%, which could be used to effectively predict the bond strength of a grouted sleeve joint. In addition, the ultimate bearing capacity of the GSCs decreased with decreases in the steel bar embedded length, grouting material saturation, and steel bar and sleeve diameter. When the steel bar embedded length and grouting material saturation decreased from 7d to 6.5d, the ultimate bearing capacity decreased by 9.4% and 2.5%, respectively, and the failure mode changed from steel bar to sleeve fracturing. Two methods, involving an increased wall thickness and a combined sleeve, are proposed for sleeve fracture defects. They effectively solve the problem of reduction in the ultimate bearing capacity owing to the increase in local stress caused by the insufficient length of the steel bar embedment and grouting material.